1. Field of the Invention
The present invention is related generally to sensorless synchronous motor drives, and more particularly to drives with startup failure detection.
2. Description of Related Art
Permanent magnet synchronous machines (PMSMs) are widely used as electric motors because of the development of high quality power electronics and powerful signal processing microprocessors. PMSM devices are typically cheaper than DC machines and can have higher performance characteristics than DC machines. PMSM devices are also attractive of their high efficiency, low maintenance costs and high torque output for a given size dimension.
The operation of a PMSM typically calls for synchronization of the rotor angle of the motor to obtain the desired output characteristics. Prior PMSMs included position sensors, such as encoders or resolvers to contribute to synchronizing the phase excitation with the rotor position. However, the presence of an encoder or resolver in many applications and environments has several drawbacks, including cost, reliability, machine dimensions and noise immunity.
A variety of techniques to detect rotor position in PMSM motors have been developed to obtain a sensorless rotor position feedback that permit the elimination of position sensors. In some cases, the sensorless rotor position detection techniques focus on the rotor saliency and local saturation of motor coils. These types of systems have internal permanent magnets for which saliency is readily measured. Rotor saliency can also be detected by injecting a high frequency sinusoidal voltage or other disturbance that is responsive to the position of the permanent magnet in relation to the motor coils. However, signal injection produces other problems related to efficiency or audible noise.
Other types of machine systems with minimal saliency like surface mounted PM motors, use back EMF to sense rotor angle. Back EMF is also observed to extract information to estimate flux in the motor coils.
Back EMF sensorless techniques for the sensorless control of a PMSM motor present difficulties at start up and during operation at low speeds. Although rotor position may be detected using the above techniques, even when the rotor is at a standstill, controlling a PMSM motor with a surface mounted PM at startup presents problems because no saliency measurement is readily available.
During a sensorless motor drive startup, motor torque is developed to overcome drive stiction and friction to rotate and accelerate the rotor. Motor shaft stiction and friction may vary dramatically dependent upon the applied load characteristics. For example, the stiction of an outdoor pump exposed to low temperatures may increase dramatically from a nominal value. In some instances, the motor shaft may be partially jammed, which can prevent motor startup rotation. In such a case, startup failure may occur, meaning that the drive torque per ampere ratio decreases dramatically and the motor is unable to accelerate. In addition, because of the difficulties with detecting rotor position in a sensorless drive at zero or low speed, the motor speed information may not be as accurate.